Re: [boost] [variant] Please vote for behavior

Subject: Re: [boost] [variant] Please vote for behavior
From: Dave Abrahams (dave_at_[hidden])
Date: 2013-01-25 18:58:55

on Fri Jan 25 2013, Neil Groves <neil-AT-grovescomputing.com> wrote:

>>
>> I think variant after move is like int without initialization:
>> int i;
>> cout << i; // don't do this
>> If the fact, that moved-from objects are only good for destruction or
>> assignment-to is accepted, then invariants for moved-from objects are
>> allowed to be violated.
>>
>>
> I completely agree with the notion that a moved-from object simply should
> not be used in any manner. 

Well, that's just wrong, for the non-destructive move model used by the
standard.  The standard library relies on the ability to both assign
and destroy moved-from elements.  If you want destructive move, that's a
whole research project.  We on the committee who created rvalue
references couldn't figure out how to make it work.

> It is interestingly divergent from the typical Design by Contract
> idiom. It is much more usual, in my experience for the
> class-invariants to hold up to and including entry to the destructor.

I don't understand what any of that means.

> It seems that this divergence is of little importance to users of the
> language since we are not explicitly interested in calling the
> destructor.

Indeed we are!  Trying to keep track of which elements have been
moved-from so you can avoid calling the destructor gets very hairy, very
quickly.  Again, we couldn't figure out how to do it.

> If I understand correctly the standard is divergent 

divergent from what?

> to allow sensible compiler implementations of clean-up code for
> moved-from objects.

I don't understand what you're saying here, either.

-- 
Dave Abrahams
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